The growth of thin films is a central step in the fabrication of many functional materials and devices. While film growth efforts have been traditionally directed toward films greater than 100 nm, recent trends in several areas are calling for the growth of films ranging in thickness from a few atomic layers up to tens of nanometers.
As a significant example of this trend, the semiconductor industry has mandated a continuous reduction in microelectronics feature sizes from 65 nm in 2006 to 22 nm in 2016. This push toward smaller feature sizes requires controlled growth of films as thin as 1 nm. In addition to film growth on two-dimensional substrates such as etched silicon (Si) wafers, there is an increasing call to apply film growth techniques to nanoscale three-dimensional substrates such as nonparticles, nanowires, carbon nanotubes, and biomaterials such as viruses, proteins, and natural fibers. Films on these substrates can play protective, passivating, or other functional roles, or may reproduce the shape of interesting nanoscale entities. The coatings may be as thin as a few monolayers to achieve the desired properties.
Atomic layer deposition (“ALD”) is a thin film deposition technique that addresses many of the current technological demands. In a typical ALD process, a substrate is contacted with a first chemical composition that modifies the substrate for a first predetermined period of time (a pulse). Such modification involves adsorption to the surface of the substrate, reaction with the surface of the substrate, or a combination of adsorption and reaction. A purging gas is introduced to remove any lingering first gaseous chemical composition in the vicinity of the substrate. A second gaseous chemical composition that reacts with the modified substrate surface is introduced for a second predetermined period of time into the vicinity of the substrate to form a portion of the thin film. A purging gas is subsequently introduced to remove any lingering second chemical composition in the vicinity of the substrate. These steps of contacting the substrate with the first chemical composition, purging, contacting the substrate with the second gaseous chemical composition, and purging are usually repeated a plurality of times until a film of desired thickness is coated onto the substrate. Although the prior art ALD processes work well, there is unfortunately only a limited number of chemical precursors having the requisite thermal stability, reactivity, and vapor pressure for ALD.
Accordingly, there is a need for thermally stable volatile precursors suitable for depositing thin films by atomic layer deposition.